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Do I have to predrill before running screws into fiberglass

10K views 23 replies 14 participants last post by  abrahamx 
#1 ·
I will be replacing the wood on my boat. The new wood is made up and the old wood is off. I am going to fill the old holes and then install the new wood. Do I have to drill a pilot hole into the fiberglass first? Also if anyone has done this job I will take any other advice before I start.
 
#6 ·
Dude, a chamfer (counter-sink) bit is cheap and takes only seconds for each hole.
For bedding deck hardware, butyl tape is the best thing for 99% of applications.

You can buy it from Maine Sail's website: Compass Marine "How To" Articles Photo Gallery by Compass Marine at pbase.com

He sells the best, white butyl tape. A roll will last forever. Proper application is described in text and photos on his website. If you really are in a hurry, you can buy black butyl tape at NAPA as "windshield tape" or windshield sealant.

This stuff is WAY cleaner, and easier to use than 3M 4200/5200, is more effective, and lasts longer.
 
#7 ·
I got free butyl tape at work. thought I read that it was kind of sloppy for doing wood rails. I figured I would just use a larger bit for a chamfer. My point was that I will have to pre drill the holes in the glass in able to chamfer. Meaning I can not just set the wood there and drill through both at the same time. Making it much harder to layout and line up the hole pattern on my wood to the holes in the glass. Unless I am missing something.
 
#8 ·
Lining up the holes isn't THAT hard (and I'm lazy):

Drill the holes in the wood.
Put the wood over the fiberglass and tape it in place.
Put a bit in the drill that is the same size as the holes in the wood, so you're using the hole in the wood as a guide.
Slip the bit into the hole and just kiss the surface of the fiberglass.
Repeat for the other holes.
Remove the wood.
Put the proper sized bit in the drill.
Drill the pilot holes at the point where the drill kissed the fiberglass.
Chamfer the edges of the pilot holes.
Put the wood back over the newly drilled and chamfered holes, and add the screws.
 
#10 ·
Lining up the holes isn't THAT hard (and I'm lazy):

Drill the holes in the wood.
Put the wood over the fiberglass and tape it in place.
Put a bit in the drill that is the same size as the holes in the wood, so you're using the hole in the wood as a guide.
Slip the bit into the hole and just kiss the surface of the fiberglass.
Repeat for the other holes.
Remove the wood.
Put the proper sized bit in the drill.
Drill the pilot holes at the point where the drill kissed the fiberglass.
Chamfer the edges of the pilot holes.
Put the wood back over the newly drilled and chamfered holes, and add the screws.
Oops, sorry I didn't answer your original question. Yes, this procedure seems fine. Just chamfer the holes, and bed with butyl.

Um... I'm a real stickler about water intrusion and core rot. If this area isn't solid glass, I'd over-drill every one of these holes, fill them with epoxy to seal the wood core, then re-drill them to the proper size for the screws, and bed them with butyl tape.

If the area is solid fiberglass, then just carry on with your procedure.
 
#11 ·
You will likely not be able to use the butyl along the length of the wood between the screws. I re-bedded the wood eyebrows on my cabin top last year and the screws are about 1.5' apart. Not enough force to compress the butyl properly. I used butyl around the screws and Sikaflex 291 under the wood.
 
#12 ·
Better yet.... To improve the previous instructions...

Drill the holes in the wood.
Put the wood over the fiberglass and tape it in place.
Put a bit in the drill that is the same size as the holes in the wood, so you're using the hole in the wood as a guide.

Drill through the wood and into the fiberglass.
Remove the wood.
Chamfer the hole in the fiberglass just a bit.
Replace the wood.
Install screw. (this will hold the wood in place and not allow it to move.
Drill the rest of the holes.
Remove the wood, chamfer and finish the job.


Slip the bit into the hole and just kiss the surface of the fiberglass.
Repeat for the other holes.
Remove the wood.
Put the proper sized bit in the drill.
Drill the pilot holes at the point where the drill kissed the fiberglass.
Chamfer the edges of the pilot holes.
Put the wood back over the newly drilled and chamfered holes, and add the screws.
While it might seem like a lot of extra work, I've seen (and suffered) the consequences of having stuff NOT QUITE LINE UP. And if you think it still might slip, do two holes before drilling the rest. The more holes you are making the more likely one won't line up properly. And we won't even talk about mating two surfaces that aren't flat and square to one another!
 
#13 ·
As for the size of the hole in the fiberglass, making a hole larger than usual for wood has already been suggested. There seems to be no table for pilot holes in fiberglass.

An method that has worked for me was to acquire a scrap piece of fiberglass, start with the appropriate pilot hole for hardwood and drill and test increasing larger holes until the more or less optimum hole is discovered. That hole size seemed larger than I had anticipated.
 
#18 ·
This is a good tip. After drilling the small hole in the glass use a non-pilot bit whose size is about the size of your chamfer and - with your drill in reverse - start the chamfer by smoothing the edge of the hole before using the countersink/chamfer bit. If your large bit has the appropriate face angle you may be able to complete the chamfers with the bit eliminating the use of the chamfer bit (and higher potential for chipping the gel coat).
 
#22 ·
If you are trying to use a larger drill bit as your chamfer tool you may end up with a bigger hole. Most drill bits are made to pull in so when you try to just "kiss the edge the drill pulls itself in. Sometime running a larger bit in reverse works but I learned the hard way that on fiberglass/gelcoat a chamfer bit is the only way to go.

my 2 cents

John
 
#24 ·
I'm not done yet but I am filling the old ones and drilling new ones. I'll reuse the through hole winch holes. I filled all my handrail holes. I did not take them off myself and after reading "this old boat" it says hand rails must be through hole bolts. I'm not sure if mine were or not. If they were I probably would have reused the holes although this old boat says to make new ones.
 
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